I don't think its something you can quantify. It would be better asking what is reasonable in the circumstances you were in at that time that made you do what you did to negate the threat. No one can ever say I would do this or do that because each set of circumstances would be different and each person would deal with the threat in a different way. The CPS produced a booklet about it as the media have put a certain perspective on self defence that is not correct, ie you will go straight to jail if you defend yourself.

My understanding of the law on self defence re this specific issue is as follows;

a judge and therefore a court and jury cannot dictate on what level of force is or should be considered reasonable in any given situation, hence this is why there is no arbitrary set level of force or technique that has to be used in self defence, making anything over and above that unreasonable and excessive.

this is why a judge has to direct a jury to weigh up the circumstances as the person acting in self defence honestly believed them to be at the time of the event, ie:the judge tells the jury to imagine themselves in the same situation with the same feelings and thoughts and then think what they would do.

So let’s consider a situation;

1. / someone is threatening you with violence which you honestly believe they intend to inflict upon you. You therefore act in self defence using a pre-emptive strike of whatever kind. Let’s say it a right cross. As per your training you use maximum effort in your strike it lands square on the aggressor’s jaw knocking him out cold. Before he has even hit the ground you have disengaged and are running away to safety not knowing what effect your action has had on the aggressor. You were in fear of immediate violence and acted lawfully in self defence.

2. / behind you as the aggressor hits the ground his head hits the kerb causing a fractured skull from which he later dies. This does not necessarily make you liable and can in no way be determined as excessive or unreasonable force. The courts cannot and should not attempt to examine and view the situation without regard to what was going on in your mind at the time.

3. / however had you struck the aggressor as stated above and continued striking while being aware that he was unconscious and falling to the floor then this could be considered by the courts as excessive and unreasonable but other factors could come into play here that may still cause the court to deem your actions reasonable in the circumstances.

4. / then again having continued striking the aggressor as he falls to the ground and then putting the boot into him while he is unconscious then the court would have no difficulty in finding your actions excessive and way beyond that which could be considered reasonable in order to negate the threat to you. Such action would be viewed as punishment or vengeance.

the above being the case do not be surprised if having stunned and ran as in the first two scenarios if the police do come asking questions and arrest you. Here I will refer you to a more detailed article on self defence and the law found in Jissen issue No5 at http://www.jissenmag.com/backIssues.asp

With regards to the issue of running this should be part of your self defence training not only for general fitness. For these purposes I would practice running using the following drill, from stationary, sprint at 100% effort for approx 25-50 metres then continue running at a steady state of say 50% max effort for another 50 metres minimum.